The present invention relates generally to automotive vehicle controls, and more particularly, to a transmission control that uses a cable.
Automotive vehicles provide a number of controls which allow the driver to control the various functions of the vehicle during operation. One control that is typically provided is a shift control for the transmission. The types of transmissions that are available in the vehicle art and the corresponding controls for these transmissions are numerous. Currently, however, automatic transmissions are the most popular type of transmission used in new vehicles. As is well-known by many, automatic transmissions simplify shifting of the gear speeds in a vehicle by providing an intuitive shift pattern with a limited number of control selections for park, reverse and forward speeds. This makes driving the vehicle much easier because the driver can choose a single selection and the transmission then automatically shifts the various transmission gears based on the speed of the vehicle and the torque load on the engine.
Several different types of shift controls are generally available for automotive vehicles. In the case of automatic transmission shift controls, a shift lever is generally provided which the driver operates by moving the shift lever through a straight, inline pattern. Detents and labels are usually provided at each of the control positions so that the driver can easily make the desired selections. Desirably, the shift lever should also be located at a convenient place near the driver for easy operation. For example, in some automotive vehicles the shift lever is mounted to the cab mounted steering column of the vehicle""s steering system. However, in the case of heavy-duty trucks, the shift lever is often mounted to the front instrument panel near the middle of the vehicle. In this location it is particularly desirable to provide clearance below and rearward of the shift control to allow individuals to easily move between the passenger and driver seats and to place packages on the floor below the shift control.
Designers of automotive vehicles continuously endeavor to design vehicles that are less expensive to manufacture while maintaining the performance and safety requirements that are expected by purchasers. One area in the art of heavy-duty trucks where cost reduction and improved performance is possible is the park locking system. Traditionally, heavy-duty truck manufacturers have used manually operated driveline brakes for the park locking system. In these systems, a drum brake mounted to either end of the drive shaft is operated by a separate control independent of the shift control. However, this type of park locking system is expensive and cumbersome to use since it requires a separate park locking control.
One alternative to the traditional manual drum brake system is a power assisted drum parking brake. In this type of system, the drum brake is actuated by spring force and is disengaged by pressurized air or hydraulic fluid. Since this system is usually operated by a detented hydraulic valve, the park locking control can be integrated with the shift control, thus eliminating one control. One disadvantage of this system, however, is the additional cost and complexity of the hydraulic circuitry.
Another alternative park locking system employs a park pawl locking mechanism in addition to a separate park brake. Typically, the park pawl is a pivoting arm with a number of gear teeth on one end that is installed inside the transmission. The park locking mechanism is engaged by pivoting the park pawl until the teeth are enmeshed with the teeth of one of the transmission gears, thus locking the transmission. Because the park pawl can be operated with a detented lever, the control for the park locking system can be integrated into a single shift control like the hydraulically actuated drum brake. However, unlike the hydraulically actuated drum brake, this system is considerably less expensive because the complicated hydraulic circuitry is unnecessary.
One problem with park pawl locking systems is the high forces that can be required to disengage the park pawl. Typically, this problem occurs when the vehicle rolls slightly while the park pawl is engaged. When this situation occurs, the tooth contact between the transmission gear and the teeth of the park pawl can tend to resist disengagement. Generally, this problem is exacerbated in heavy-duty trucks when they are parked on slopes because the heavy weight of the truck transfers a large amount of torque to the transmission gear. As a result, the force required to move the shift lever out of the park selection in an integrated control can be considerably higher than any of the other shift selections. Accordingly, in one example the shift lever force required to disengage the park locking system can be as high as 45 lbs., whereas the shift lever force is only about 5 lbs. for the other shift selections.
One problem with high park pawl disengagement forces is that it can cause the shift control cable to wear out and fail prematurely. The shift control cable is typically connected at one end to the cab-mounted shift control and is connected at the other end to the transmission shifting mechanisms. Accordingly, the shift control cable translates the movement of the shift lever to the transmission. Typically, cables are rated to transmit only small amounts of force while in compression to prevent premature failure of the cable. Therefore, a shift control that minimizes compressive loads on the shift control cable is desirable.
Accordingly, an integrated shift control is provided for shifting the gears of an automatic transmission and actuating a park pawl locking system. The shift control includes a lever portion attached to a handle. The lever portion is pivotally attached to the body of the shift control. One end of a cable is then pivotally attached to the lever portion between the handle and the lever portion pivotal attachment. Accordingly, the shift control pulls the cable, placing it in tension, when the handle is moved out of a park selection. Relative small forces are applied to the cable when the cable is pushed in compression, thus preventing cable overload and premature failure.